Spiral antennas that are equi-angular or are Archimedean spirals, are known to be highly useful in very high frequency applications. A planar spiral antenna, for example, has two branches lying in the same plane that are symmetrical with respect to a point and are in phase opposition to produce maximum radiation in two directions that are symmetrical with respect to the antenna plane. Such antennas are widely used in air-borne direction finding systems and commonly radiation in a single direction from such an antenna is obtained by backing the spiral with a coaxial cavity having a depth that is equal to one-quarter wavelength at average operating frequency of the antenna. This is intended to cause reflected radiation in the cavity and direct radiation in the same direction from the spiral to be in phase in the plane of the spiral to reinforce rather than interfere with radiation; however, the foregoing is only true over a limited frequency range.
In order to improve or extend the frequency range of spiral antennas, it is common to provide various types of cavity structures backing the spirals. As an example of the foregoing, reference is made to U.S. Pat. Nos. 3,192,531, 3,358,288, 3,441,937, 3,555,554 and 3,686,674.
The low frequency cutoff for a spiral antenna is a function of the spiral antenna size and it has been found that a 2-1/2 inches diameter spiral antenna operates satisfactorily down to about 2 GHz. It would be desirable to extend the low frequency range, particularly for aircraft direction finding, without increasing the antenna size because of the difficulties in mounting larger sized antenna on aircraft structures. In order to extend the low frequency range down to 500 MHz, for example, it would normally be necessary to increase the antenna diameter to about 10 inches and in addition to the undesirability of this increase in size, added design problems are encountered, such as moding.
The present invention provides a second antenna inside of a spiral antenna cavity to materially extend the low frequency range of the antenna without increasing the size thereof.